This research will test a simple, plausible, and novel proposal about how the cerebellum makes voluntary rapid eye movements (saccades) accurate. We propose that saccade accuracy depends on a signal that stops saccades at the right time. This signal travels across the midline of the cerebellum in the axons called parallel fibers (p-fibers). The arrival time of this signal, and consequently the onset of the deceleration that makes saccades accurate, is set by the amount of time it takes the signal to travel from one place in the cerebellum, across the midline, to another part on the other side. We will test this proposal by cutting saccade-related p-fibers. If this idea is right, then the lesion will: 1) abolish the cerebellum signal that helps stop saccades on time;2) make saccade deceleration abnormally slow;3) make saccades overshoot their targets. Preliminary data show that the lesion causes all three predicted effects. A corollary of our proposal is that large saccades are represented laterally in the saccade part of the cerebellum and small saccades are represented medially. This is consistent with earlier findings. We will use anatomical tracing to describe the connections from lateral and medial parts of this region to determine how these connections cause movements of different sizes. Preliminary data show that the lateral and medial areas of the saccade part of the cerebellum make very different connections. Describing this will tell us how the cerebellum makes movements of different sizes. If, like our pilot results, the work proposed here supports our proposal, then it will provide a basic framework for how the cerebellum transforms the saccade-related signals that it receives into the signals that it uses to make saccades accurate. This work will also show why saccades, and consequently vision, are badly impaired by surgery to remove the most common brain tumors in children. This will motivate and guide easy-to- implement modifications of treatment and surgery to remove tumors without impairing vision. Such improvements could help patients quickly because they require no other innovations. PUBLIC HEALTH RELEVANCE The proposed work will explain why eye movements, and consequently vision, are badly impaired in children after surgery to remove the most common brain tumor in children. This explanation will motivate and guide improvements in the surgery so that it removes these tumors without impairing vision. In addition, this work will provide the clearest description yet of how the cerebellum transforms input into signals that make movement, in this case eye movement, accurate.